The operation of an internal combustion engine, such as, for example, a diesel, gasoline, or natural gas engine, may cause the generation of undesirable emissions. These emissions, which may include particulates and oxides of nitrogen (NOx), are generated when fuel is combusted in a combustion chamber of the engine. An exhaust stroke of an engine piston forces exhaust gas, which may include these emissions, from the engine. If no emission reduction measures are in place, these undesirable emissions will eventually be exhausted to the environment.
Research is currently being directed towards decreasing the amount of undesirable emissions that are exhausted to the environment during the operation of an engine. It is expected that improved engine design and improved control over engine operation may lead to a reduction in the generation of undesirable emissions. Many different approaches such as, for example, engine gas recirculation and aftertreatments, have been found to reduce the amount of emissions generated during the operation of an engine. Unfortunately, the implementation of these emission reduction approaches typically results in a decrease in the overall efficiency of the engine.
Additional efforts are being focused on improving engine efficiency to compensate for the efficiency loss due to the emission reduction systems. One such approach to improving the engine efficiency involves adjusting the actuation pattern of the engine valves. For example, the actuation pattern of the intake and exhaust valves may be modified to implement a variation on the typical diesel or Otto cycle known as the Miller cycle. In a “late intake” type Miller cycle, the intake valves of the engine are held open during a portion of the compression stroke of the piston. Implementing a pattern variation, such as the late-intake Miller cycle, may improve the overall efficiency of the engine.
The engine valves in an internal combustion engine are typically driven by a cam arrangement that is operatively connected to the crankshaft of the engine. The rotation of the crankshaft results in a corresponding rotation of a cam that drives one or more cam followers. The movement of the cam followers results in the actuation of the engine valves. The shape of the cam governs valve lift during valve actuation. The relationship of valve lift to cam angle, as dictated by the shape of the cam, creates a predetermined engine valve actuation pattern as the cam is rotated.
An engine valve actuation system may include a hydraulic actuator that is adapted to vary the valve actuation pattern established by the shape of the cam. For example, as described in U.S. Pat. No. 6,237,551 to Macor et al., issued on May 29, 2001, an engine valve actuation system may include a hydraulic actuator that establishes a hydraulic link between the cam and the intake valve. When the link is established, the valve will be actuated according to the shape of the cam to produce a predetermined valve actuation pattern of valve opening and valve closing. However, when the hydraulic link is broken, such as by opening a control valve, the force of a valve return spring causes the engine valve to close. Thus, breaking the hydraulic link allows the engine valve to move through an actuation pattern different from the predetermined pattern that would be achieved by the shape of the cam.
However, the operational performance of the hydraulic actuator may depend upon the viscosity of the operating fluid. When the operating fluid is cold, such as when the engine is starting, the hydraulic actuator may experience slow response times. The slow response times may lead to the engine experiencing rough running conditions or difficulty starting until the operating fluid is warmed enough to allow the hydraulic actuator to operate properly. Depending upon the current environmental conditions, the engine may need to operate for a period of time to warm the operating fluid so that the hydraulic actuator will operate as expected.
In addition, a hydraulic actuator, such as described in the '551 patent to Macor, may not be able to actuate the engine valve independently of the cam assembly. The force exerted by the engine valve springs may be insurmountable with a hydraulic actuator that is limited by the pressure and velocity of fluid supplied by a low pressure fluid supply system. An increase in the demand for force or response time could require additional capacity of the fluid source, typically resulting in an increase in the cost of the engine as well as a decrease in overall efficiency of the engine.
The engine valve actuation system of the disclosed invention solves one or more of the problems set forth above.